Electrically conductive, thermoplastic polymer compounds are of increased practical interest, for instance, for packaging electronic instruments and parts, and to solve a wide range of static decay, static shielding and electromagnetic shielding problems. Often such compounds are made by mixing, for example, carbon black, stainless steel fibers, silver or aluminum flakes or Nickel-coated fibers with insulating bulk thermoplastics such as polystyrene, polyolefins, nylons, polycarbonate, acrylonitrile butene styrene (ABS) copolymers, etc. These filled compounds are subsequently processed into the desired shapes and articles by extrusion, injection or blow molding and the like. Major problems associated with the above filled thermoplastic compounds are that processing of these materials is not trivial, is often associated with excessive machine wear and that their colorability is difficult due to the mechanical and optical properties of the fillers, respectively. For example, it is virtually impossible to produce carbon black filled polymers having a high electrical conductivity that are not black. The importance of colorability of conductive compounds derives from applications of these materials in, for instance, the carpet and fashion industry where product appearance is critical, and for color coding of films, containers, housings and enclosures.
More recently, there has been an increased interest in replacing the carbon black or metal filled compounds with intrinsically electrically conductive polymers and their blends with common insulating polymers. The latter systems are believed to be more cost competitive, easier to process and to exhibit desirable mechanical properties. Among the various conductive polymers, the polyanilines have attracted particular attention because of their excellent environmental stability and their low production costs.
Polyaniline is well known in the art, and the preparation of the electrically conductive form of this polymer based on, for example, contacting polyanilines with protonic acids has been disclosed. Green, A. G., and Woodhead, A. E., "Aniline-black and Allied Compounds, Part 1, "J. Chem. Soc., Vol. 101, pp. 1117 (1912); Kobayashi, et al., Electrochemical Reactions . . . of Polyaniline Film-Coated Electrodes," J. Electroanl. Chem., Vol. 177, pp. 281-91 (1984); U.S. Pat. Nos. 3,963,498, 4,025,463 and 4,983,322; U.S. patent application Ser. No. 714,165. Typical examples of such disclosed protonic acids are HCl, H.sub.2 SO.sub.4, sulfonic acids of the type R.sub.1 -SO.sub.3 H, phosphoric acids, etc. Chiang, J. -C. and MacDiarmid, Alan G., "Polyaniline: Protonic Acid Doping of the Emeraldine Form to the Metallic Regime", Synthetic Metals, Vol. 13, p. 196 (1986); Salaneck, W. R. et al., "A Two-Dimensional-Surface "State" Diagram for Polyaniline" Synthetic Metals, Vol. 13, p. 297 (1986). Such acids form complexes with polyaniline, which, generally, exhibit electrical conductivities of 10.sup.-3 S/cm or more. Thus, the electrical properties make these so-called "doped" polyanilines and their blends and compounds with common insulating bulk polymers suitable for a variety of the anti-static and shielding applications that are currently served by metal or carbon black filled systems. Indeed, certain polyaniline-based systems may be conveniently processed using standard polymer processing techniques without machine wear and exhibit excellent mechanical properties.
However, invariably the polyanilines in the art in their conducting form show very strong absorptions around 300-400 nm (which corresponds to about 3.2 eV) and from 600 nm upwards (which corresponds to 2.0 eV downwards) in the visible spectral range, giving the polymers an intense, dark, black/green/blue appearance; cf. the absorption spectrum in FIGS. 1 through 6.
This very dark blue/green/black color of conventional conductive polyanilines is well recognized. (See, for example the text of Examples 2 and 8 of Patent Cooperation Treaty patent application serial number WO 90/10297.) Polyaniline rendered conductive through protonation with commonly used protonic acids exhibit about the same absorption spectrum, and, hence, the same blue/green/black color. Examples of conductive acid-doped polyanilines which will exhibit this characteristic blue/green/black color are those based on the following dopants:
______________________________________ DOPANT REFERENCE ______________________________________ p-toluene sulfonic acid WO '297-text Example 8; S. K. Dhawan et al., Polym. International, 25, 1, 55 (1991). 1,5-naphthalenedisulfonic WO '297 - text Example 2. acid, tetrahydrate benzene sulfonic acid S. K. Dhawan et al., Polym. International, 25, 1, 55 (1991). sulfosalicylic acid K. Tzou et al., Synthetic Metals, 53, 365 (1993) HCl M. G. Roe et al. Physical Review Letters, 60, 2789 (1988); P. M. McManus et al., J. Physical Chemistry, 91, 744 (1987); K. Tzou et al, Synthetic Metals, 53, 365 (1993) H.sub.2 SO.sub.4 E. M. Genies et al., J. Electroanal. Chem., 220, 647 (1987) HClO.sub.4 M. Enoue et al., Synthetic Metals, 30, 199 (1989) HNO.sub.3 Y. Li et al., Synthetic Metals, 25, 79 (1988) acetic acid A. Ray et al., Synthetic Metals, 29, E141 (1989) camphor sulfonic acid (CSA) See FIGS. 2 and 3 dodecylbenzene sulfonic See FIG. 3 "PANI-DBSA" acid 1,5-naphthalenedisulfonic See FIG. 4 acid butylsulfamic acid See FIG. 4 hydroxyaminosulfonic acid See FIG. 4 4-nitrotoluene-2-sulfonic See FIG. 5 acid m-xylene-4-sulfonic acid See FIG. 5 2-acrylamido-2-methyl-1- See FIG. 5 propanesulfonic acid 2-naphthalene sulfonic acid See FIG. 6 4-hydroxy-3-nitroso-1- See FIG. 6 naphthalene sulfonic acid 2-sulfobenzoic acid See FIG. 6 sulfoacetic acid See FIG. 6 2-hydroxy-4-methoxy- See FIG. 7 benzophenone-5-sulfonic acid p-chlorobenzene sulfonic See FIG. 7 acid phenylhydrazine-p-sulfonic See FIG. 7 acid ______________________________________
The conductive polyanilines disclosed by Cameron in U.S. Pat. No. 4,935,163 and by Tamura et al in U.S. Pat. No. 4,556,623 employ dopants which are chemically so similar to these materials that they would yield very similar absorptions and hence similar blue/green/black colors.
From the spectra provided herein as FIGS. 2 through 7 it may be readily seen that the protonated polyanilines of the art are characterized by two strong absorptions, one around 350-400 nm and one from about 600 nm upwards. It can also be seen that the absorption at 400 nm generally is about as strong as that at 850 nm.
Clearly, selection of specific protonic acids to yield conducting polyanilines characterized by distinctly different absorption spectra in the visible range, and therefore colors which range outside of the dark blue/green/black palette of the past, would be desirable. Due to the relatively low conductivity of doped polyaniline, in comparison with silver, steel and other metals, often large amounts of the polymer are needed in compounds or blends with insulating polymers to impart sufficient conductivity for the desired shielding and antistatic applications. Thus, much like the metal and carbon black filled thermoplastic compounds, the colorability of the polyaniline-based systems is believed to be poor and problematic. Clearly, a need exists for methods to fabricate colored, electrically conductive compounds and articles.